“It appears that the pressure from the well caused the explosion at the wellhead,” said Alan Krenek, chief financial officer at Basic Energy, which relocated its headquarters last year to Fort Worth from Midland.

The job involved a workover rig, which goes back into a well to boost production.

Here are the names the men that the sheriff’s office reported as dead and injured: Juan Rios died at the scene; Olegario Rios died at Medical Center Hospital, where medical personnel airlifted him; Roman Galindo was in critical condition and Jose Cruz was in serious condition after they ended up airlifted to University Medical Center in Lubbock, and Don Owen, a consultant on the location, ended up treated and released at Reeves County Hospital for gas inhalation.

All men’s ages and home were not immediately available from the sheriff’s department.

The well ended up shut in by crews on site, the sheriff’s department said. The cause of the incident remained under investigation and officials with Occupational Safety and Health Administration were on the scene.

Basic Energy provides drilling, pumping and fluid services to help oil and gas companies like Anadarko produce from wells. The company has about 5,600 workers in Texas, Louisiana, Oklahoma and New Mexico and other states, according to a Feb. 27 regulatory filing.